This invention is generally related to wireless communications devices, and more particularly to multi-band antennas for wireless communications devices.
Wireless communications devices such as cellular phones, personal communication service (xe2x80x9cPCSxe2x80x9d) phones, pagers, and cellular modems are increasing in popularity and becoming ever more prevalent. Not only are the number of wireless communications devices increasing, but also the variety of devices and the types of available services are increasing. For example, many wireless communications devices now offer data services such as Internet access, in addition to voice and/or text messaging services.
Wireless communications devices typically employ one or more antennas and a receiver, transmitter or transceiver for providing wireless communications. These devices operate by emitting and/or receiving radio frequency (RF) radiation at a variety of frequency bands of the electro-magnetic spectrum. Reference herein to RF radiation and/or RF signals refers to operation in any portion of the electro-magnetic spectrum suitable to wireless communications, not only the portion typically associated with the AM and FM radio bands. For example, cellular operation typically occurs in the 800-900 MHz range and PCS operation typically occurs in the 1.85-1.99 GHz range.
While wireless communications devices offer their users considerable convenience, current devices suffer from a number a possible drawbacks. For example, some have expressed concern regarding possible adverse effects from radiation, particularly where the wireless communications device is located close to the user""s head or body when in use. Antennas such as multi band dipole or asymmetric dipole antennas have an omni-directional free space radiation pattern, providing as much radiation in a front direction (i.e., toward the user""s head) as it provides in a back direction (i.e., away from the user""s head). Multi-band antennas (PIFA) provides little or no directivity, thus similarly exposing the user to undesired radiation levels.
Wireless communications employ a variety of operating protocols and frequency bands. The ability of a wireless communications device to employ more than one operating protocol and/or frequency band is important to the success of the device in the marketplace.
The size of wireless communications devices is important to their acceptance in the marketplace. The size is in part, a function of the number, size and shape of the antennas used for wireless communications.
In one aspect, a compact multi-band resonator is designed for internal mounting within a wireless communications device, for example, on one side of and near one end of the printed circuit board of the wireless communications device. The relatively small size of the resonator permits it to be integrated within the interior region of a wireless communications device such as a cellular phone. The resonator may have one or more curved edges that conform to a curved top edge of the plastic housing of a wireless communications device. The resonator is fed against, and works in conjunction with, a second planar conductor formed, for example, by the ground traces of the printed circuit board to form a moderately directional antenna with dipole gain. For example, directivity exhibited when tuned for the cellular and PCS bands may be on the order of 3 dB far field front to back ratio in the low frequency (cellular) band, and 7 dB in the higher frequency (PCS) band. This directivity may result in a reduction in the near field, thereby reducing the specific absorption rate (xe2x80x9cSARxe2x80x9d) when the antenna is installed on the top rear of a wireless communications device such as a cellular phone operated near the head in the talk position. The antenna structure can include a feed point that presents a 50 ohm unbalanced impedance for connection to the wireless communications device""s transmit/receive circuitry via a single hot conductor and a single ground conductor.
In another aspect, each of the frequency bands of the multi-band antenna are separately tunable. At least one discrete capacitance between a resonator and a ground plane conductor can be adjusted to tune the frequency band of the antenna. In another aspect, the higher frequency band may be tuned without affecting the lower frequency band.
In a further aspect, at least one capacitance is remotely adjustable. The capacitors can be made variable by techniques such as switched fixed capacitors which are selected by PIN diodes or by using voltage-controlled capacitors (xe2x80x9cvaractorsxe2x80x9d). In either case, the capacitance value may be controlled electrically or by a digital command signal. The command signal may originate at a site remote from the wireless communications device, such as a cell site or base station, which facilitates seamless roaming across cellular service regions having different frequency allocations for particular bands, as an example.